Question for citizen (7)
I believe the answer is:
This clue is a double definition.
'question' is the first definition.
'citizen' is the second definition.
(I've seen this before)
'for' acts as a link.
(Other definitions for subject that I've seen before include "Musical theme" , "Branch of knowledge" , "Topic being discussed" , "Division of curriculum" , "Theme (of a fugue)" .)